The Seal of Ascension is a ring with several abilities that is obtained through a quest. It is no longer required for access to the upper portion of Blackrock Spire (BRS), but it can be worn to protect you from one of Gyth's special attacks as well as summon the spirit of Vaelastrasz to help you in the fight against Rend Blackhand.

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In order to begin the quest, one must acquire an [Unadorned Seal of Ascension]. The seal is a random drop off of mobs both inside and outside the BRS instace. The seal can be purchased and sold on the Auction House.

Once one has the Seal, enter Lower Blackrock Spire (LBRS). Go up the stairs, take a right. Jump down and through the passage on the left. There will be a wooden bridge ahead of you.

Looking up on the left side, the group will see a single mob kneeling on a ledge. Walk to the end of the ledge, following it around and up an incline. What appears to be a regular mob is actually Vaelan in disguise. Upon coming into his aggro radius, he will cast a harmless spell on the group, allowing all to see Vaelan in his human form. Any group member with an [Unadorned Seal of Ascension] in their inventory will gain access to the Seal of Ascension quest.

Travel to the Wyrmbog in Dustwallow Marsh. Once inside, locate Emberstrife. Attack Emberstrife until the "Emberstrife's will is starting to falter" status message appears. Then place the unforged seal on the ground, right in front of him, where the tank stands. Use the Orb of Draconic Energy on Emberstrife. Emberstrife is now under your command for a brief time. Cast his first spell to breathe "Flames of the Black Flight" on the seal. Move him away over the pool to break Mind Control, then loot the [Forged Seal of Ascension] from the seal.

Return to Vaelan in LBRS. Hand in the quest and you will then receive the [Seal of Ascension].

Currently, there is a bug with this quest, and you will be unable to turn it in unless you have a [Unadorned Seal of Ascension] on you. Either farm mobs until another one drops, or check the AH for one.
Either this has been fixed, or it only happens when your instance has reset. If you complete the forging of the seal and return to Vaelan rightaway, he will let you complete the quest.

At some point during the battle with the false warchief, Rend Blackhand, you can use your Seal of Ascension to summon a shade of Vaelastrasz (looks like a ghostly red dragon), who will heal your entire group for 5000 HP as well as 'help' you in your fight against Rend. To do this, you just 'use' your Seal like you would any other item with a "Use: do something" tag on it. This was once an unlisted "hidden" ability, but it was since been added to the tooltip and is now quite obvious.

The tricky part is the timing of when you "use" it. One tip is to add the ring to your action bar to keep fumbling to a minimum. As soon as Gyth dies use your ring and it should work properly.

The summoning of Vael is slightly bugged in that sometimes he will spawn and charge off into the subsequent room (the one with Awbee). He won't aggro anything, although he can take up to 30 seconds to come back to the scene of the fight, by which stage it can be almost finished and thus rendering him largely useless. I tested it secretly during several UBRS PUGs and it seems to bug the most when you cast it early right before Rend dismounts. I never had it bug when using it just as Rend dismounted.

If multiple people have a Seal of Ascension, can the dragon be summoned twice?

No. Attempting to use the ring a second time, even by a different person, has no effect.

"Chromatic resistance" given to you when you "use" the ring? Does this only work when you summon Vaelastrasz or can you use it anywhere in the world?

Doesn't appear to do anything when used in Thunder Bluff

Chromatic resistance is the name of several subtypes of buff you gain when wearing the ring, and get hit by one of Gyth's attacks. Depending on the type of the attack, you gain resistance to it for a brief period of time. You can have multiple buffs of CR, each one protecting against a different magic class, but the buff only lasts for 30 seconds.